It is already known to plan, dimension and design an electrical power system for normal as well as disturbed operation by using dimensioning criteria, set in advance, for various system quantities, such as operating voltage and mains frequency.
The voltage levels of the electrical power system at different network points or nodes constitute a measure of the condition of the electrical power system, that is, the ability of the system to continue to supply the loads of connected areas with the desired power. During normal operation, the operating voltage at a network point or a node is to lie within a pre-allowed interval, normally within a few percent of the stated nominal voltage. Examples of normally occurring nominal voltages are 400 kV, 130 kV and 50 kV. The construction, function, operation, automatic and protective equipment of an electrical power system are described, as far as Swedish conditions are concerned, in our Swedish patent SE 0101061-0 entitled “An electric power plant with means for damping power oscillations”.
In case of disturbances in the electrical power system, preferably when the transmission capacity is weakened or decreasing, but also in the event of loss of electricity production plants or loss of other components with a voltage-controlling function, and combinations of these, the voltage distribution in the electrical power system is changed in such a way that the voltage level across the loads drops, which leads to a drop also of the voltage level in the transmission network. The corresponding phenomena may also arise in case of too fast a load increase.
Common in both cases is that the electrical power system is not capable, in the steady state, to supply the load connected to the network. Unless measures are taken, the different component-protective functions of the electrical power system, which are adapted to react on changes in voltages and/or currents, will successively disconnect components in the electrical power system. Normally, disconnection of transmission lines is the first thing that happens. Disconnection of components may occur over time, from a few seconds up to several hours.
A typical scenario is that the voltages at the measuring points of the impedance-measuring line protection device decrease and the currents increase, whereby the most sensitive measuring zone of the protection device disconnects the line in question. The remaining lines, in the network thus weakened, are then further loaded, whereby more transmission lines are disconnected. This proceeds until the remaining electrical power network is able to maintain a constant electricity operation and achieve a balance between production, transmission capacity and load.
Examples of events such as those mentioned above are the major power failures in the USA/Canada, Sweden and Italy in 2003. Scenarios such as those described above are, of course, desirable to avoid and there are a plurality of methods to detect incipient voltage instability in an electrical power system. In such systems, voltages and reactive power flows are measured, and devices for limiting the current on generators for detecting voltage instability have also been used, see, for example, Ingelsson, Karlsson, Lindström, Runvik, Sjödin: “Special Protection Scheme against Voltage Collapse in the South Part of the Swedish Grid”, CIGRE Conference, Paris, 1996.
Still more advanced detectors, with or without possibilities of communication, have been presented and tested. One such example is the so-called VIP algorithm (Voltage Instability Predictor) described in Begovic, Milosevic, Novosel: “A Novel Method for Voltage Instability Protection”, Proceedings of the 35th Hawaii International Conference on System Sciences, 2002. This system compares the impedance at a certain network point, in the direction of the load, with the impedance in a direction towards the production source. The relation between the load impedance, thus measured, and the source impedance may then be used as a measure for detecting incipient voltage instability.
One common method, which is efficient in this context, for mitigating or preventing voltage instability, after such instability has been detected, is to disconnect parts of the load in the network. By disconnecting a load corresponding to some ten percent of the total load in the inflicted area, the remaining load may often be adequately supplied. By intentionally and in a controlled manner disconnecting some load objects in a stressed operating situation, at least the operation of the major part of the inflicted area is saved, while at the same time the transmission network is kept energized and intact. The disconnected loads may be energized more rapidly again when stability has been resumed, for example by switching operations in the network and start-up of local electricity production.
By selecting and determining in advance what load objects are to be disconnected and in what sequence, a possibility is obtained of selecting objects such that the harmful effects in the system are limited. A frequency-controlled load disconnection for disconnection of load objects at too low or decreasing frequency is already available in most electrical power systems, that is, when the balance between actual electricity production and the desired consumption has been disrupted, for example in case of a major loss of production.
Other measures than load disconnection, such as the connection of shunt reactors and shunt capacitors, the connection of emergency power via dc links, etc., may also be used.
Existing non-connected electric power-generating objects cannot be connected to the transmission network if the line voltage is lower than a lowest value given in advance. For this reason, it may be necessary to disconnect part of the load such that reserve power plants may then be connected. These reserve power plants may then be used to increase the reactive generation and additionally increase the line voltage. In addition, they are to deliver active power and make it possible again to connect the previously disconnected load without the balance between production and consumption being lost.
With a view to designing a selective protection against voltage instability, based on the use of load disconnection, primarily three questions need to be answered in view of the prevailing operating situation:                1) When should the load disconnection take place?        2) Where should the load disconnection take place?        3) How much load should be disconnected?        
The voltage level is normally a sufficiently good criterion for determining where the load disconnection should take place.
Disconnection may take place “without unnecessary delay”, that is, as quickly as possible after a margin has been allowed for reserve disconnection of a shunt fault, that is, short circuits and ground contacts. The speed should also be adapted to the speed of the on-load tap changer control of the power transformers connected in the vicinity thereof. The time delay should normally be a few seconds.
However, the extent of the load disconnection is more difficult to determine. One method is to proceed by trial and error by disconnecting one or more loads, load areas, at a time. This is a relatively slow method since the system response to each disconnection must be awaited.
Another method is to use a comfortable margin and disconnect a larger number of loads. The disadvantage of such a method is that an unnecessarily large part of the load is disconnected and also that this involves a risk of obtaining high voltages in the electrical power system.
At present, there is no known method of dimensioning or determining the magnitude of a necessary load disconnection, when the need of such a measure has been detected. The fulfilled criteria normally result directly in load disconnection via a preselected circuit-breaker function with a suitable time delay, typically a few seconds.
U.S. Pat. No. 6,219,591, “Voltage instability predictor (VIP)—method and system for performing adaptive control to improve voltage stability in power systems”, shows a method for detecting that the power system, or parts thereof, is/are heading for instability. However, the method only mentions the need of load disconnection in general, as a suitable measure for how this threatening instability is to be cancelled or mitigated.
The invention according to U.S. Pat. No. 6,242,719, “Applications and methods for voltage instability predictor (VIP)”, is a further development of the invention according to U.S. Pat. No. 6,219,591, which, among other things, relates to masked transmission networks. However, none of these publications suggests any measure of magnitude of that quantity of load disconnection that is required for stabilizing the system.
Several patent specifications deal with the VIP algorithm, which suggest stability measures of various kinds and predict an imminent voltage collapse, but do not suggest the extent of or magnitude of the load disconnection that is necessary to prevent a power failure or a collapse in the electrical power network.